The invention relates to an apparatus for treating thermoplastic synthetic plastics material, for example waste, in particular of polyester, comprising a receptacle for material to be treated to which this material is supplied through an intake opening and from which the material is delivered by at least one worm connected to the sidewall of the receptacle, at least one tool having working edges acting comminuting and, respectively, or mixing on the material disposed in the bottom region of the receptacle and being rotatable around a vertical axis, the intake opening of the worm being disposed at least substantially at the level of the tool, and at least one line connected to the receptacle for creating a vacuum within the interior of the receptacle and, if desired, for supplying a gas therein.
Such apparatus are known (EP-A 390,873), and generally they work satisfying, also when treating such kinds of synthetic plastics material which are sensitive to air oxygen and, respectively, or humidity, because by evacuating the receptacle or by supplying a protective gas into the interior of the receptacle, the synthetic plastics material can be protected against these detrimental influences.
However, it has been shown that in some cases the degree of homogenizing of the synthetic plastics material delivered by the worm is not sufficient, in particular with respect to the obtained drying degree of such synthetic plastics material which must be completely dry already before plasticizing in order to avoid downgrading, for example polyester. Polyester elements, for example foils of greater thickness, require an increased effort in drying which increases with the thickness so that for such material separate drying processes, for example with dehydrated air in special drying apparatus are required. Further, these drying apparatus operate in a temperature region that is admissible for crystalized material only, however, amorphous material would get sticky so that it would agglomerate. This means that a crystallizing process must antecede the drying process. However, if the material to be treated is treated for a long time in the receptacle, then at a continuous operation of the apparatus there occurs the danger that single synthetic plastics material elements are already clutched by the delivery worm already at a very early moment, however other synthetic plastics material elements only at a very late time. Those synthetic plastics material elements which were taken along early, may still be comparatively cold and, therefore, not pre-treated to a sufficient amount, so that the said inhomogenities in the material occur that is supplied by the worm to the connected tool, for example to an extruder head.
The invention has at its object to avoid these disadvantages and to substantially improve the homogenity of the delivered material, by using the vacuum effect within the receptacle in an advantageous manner. The invention solves this task by the features that the delivery opening of at least one further receptacle is connected to the intake opening by means of a tube element, in which receptacle also at least one tool rotating around a vertical axis is disposed in the bottom region of the receptacle, and that in the tube element a shut-off means for pressure-tight closing of the tube element is disposed for maintaining the vacuum within the interior of the receptacle provided with the line. Therefore, within the inventive apparatus two or more receptacles are disposed in a series and the synthetic plastics material to be treated must run through these receptacles one after the other. Within the first receptacle, already a pre-treatment of the synthetic plastics material takes place by a pre-comminuting, pre-heating, pre-drying and pre-compression, so that pre-homogenized material is created which is supplied to the following receptacle. Thereby it is ensured that no untreated (cold, not compressed, not comminuted or inhomogeneous) material is directly supplied to the delivery worm and via this to the connected extruder or the like. By means of the shut-off means within the tube element connecting the receptacles, a complete and reliable tightening between the two receptacles can be obtained so that losses in the vacuum or in the protective gas within the following receptacle are completely avoided as long as the shut-off means remains closed, that is, as long as the pre-treatment takes place within the preceding receptacle. Therefore, the advantages of the pre-treatment can be maintained also if within the second or within a following receptacle a vacuum treatment or protective gas treatment of the thermo-plastic synthetic material takes place. The vacuum of this treatment is even used for an increase of the mixing effect and, therefore, for improving the homogenity of the material delivered from the downstream disposed receptacle, because, when the shut-off means is opened, the vacuum within the downstream receptacle suddenly sucks on the material disposed within the upstream receptacle so that an abrupt mixing takes place within the downstream receptacle by vacuum influence.
Particularly favourable results are obtained if according to a further embodiment of the invention the exit opening of the further receptacle (that is the upstream receptacle) is disposed at least substantially at the level of the tool within this receptacle, that is within the bottom region of the receptacle, because then the said sucking-off of the material disposed within the upstream receptacle can take place without a substantial deviation thereof. Further, if the shut-off valve is opened when the tool still rotates, the tool rotating within the upstream receptacle delivers by centrifugal action into the exit opening so that the material pre-treated within the upstream receptacle is conveyed into the downstream receptacle within the shortest possible time. As soon as the shut-off valve is closed again, a fresh batch can be introduced into the upstream receptacle and can be pre-treated there.
Within the EP-A 496,090, which has a better priority, however is not pre-published, is has been proposed to mount two receptacles provided with rotating comminuting tools for the treatment of synthetic plastics waste material in a series. The material is introduced into the second receptacle by means of a conveyor worm from the first receptacle. However, such a conveyor worm cannot be made pressure-tight.
Within the spirit of the invention, in the simpliest case the shut-off means may comprise a gate plate which is closed as soon as the vacuum treatment or the gasification takes place within the downstream receptacle. However, then no continuous operation is still possible, to the contrary, the downstream receptacle must be charged batchwise. If, however, according to another embodiment of the invention the shut-off means is a sluice, in particular a cellular wheel sluice, the said tightening between the two receptacles is maintained and nevertheless a continuous operation is possible. In order to avoid that too great vacuum losses occur through the cells of the sluice, these cells can also be evacuated or a gas can be introduced therein.
As already mentioned, the vacuum created within the downstream receptacle enhances sucking-in of the material to be treated from the upstream receptacle. Within such plants, therefore, the receptacles as a rule can be disposed at the same level. However, if it is desired to improve filling of the downstream receptacle or mixing the material therein by influence of gravity, according to a further embodiment of the invention the receptacle disposed upstream, when seen in flowing direction of the material, may be disposed at a higher level than the following receptacle. The latter, therefore, can also be charged within its middle section or in its upper section of its sidewall, and optionally also through the cover from above.
Within the spirit of the invention, it has been shown that it is particularly favourable to provide the exit opening of the, when seen in flowing direction of the material, upstream receptacle with an enlargement at the arriving side of the tool. This enlargement ensures that the material can better flow into the flow cross section. According to the invention, an analogous improvement is obtained at the exit side of the transition cross section, if the intake opening of the, when seen in flowing direction of the material, downstream receptacle has an enlargement at that side at which the tool runs off. Within the spirit of the invention, the enlargements may be of pocket-like configuration, preferably having a sickle-shaped cross section, in order to ensure a smooth introduction or delivery of the material into the receptacle or out of it. dr